Michaelis is George Wilson's friend who owns a coffee shop next to the ashheaps. In Chapter 7, as Tom, Nick, and Jordan (and Daisy and Gatsby in the "death car") travel to Long Island, Michaelis stops at Wilson's garage. He finds Wilson looking sick. Wilson remarks that he's got Myrtle locked upstairs and that they're going away two days later. George Wilson is evidently (finally) upset with Myrtle. Either, he had become fed up with the way she treated him or he had realized that she was cheating on him.

Michaelis leaves George to attend to some customers. Shortly after this, Myrtle rushes into the street and is killed by the "death car." Michaelis was one of the first to rush to Myrtle's side and he became the main witness of the accident. Following the accident, Michaelis stayed with George to comfort him until the next morning.

In Chapter 8, while Michaelis is waiting with George, George is vague but he tells Michaelis that he believes Myrtle was murdered. George also told Michaelis that he would find out who the driver of the car was. George had assumed that the man driving the car was also the man Myrtle was having an affair with. George Wilson then took steps to discover the owner of the yellow car and decided to take revenge on that owner (Gatsby). We (readers) learn all of this information from Michaelis and from his and George's conversations:

The police, on the strength of what he said to Michaelis, that he “had a way of finding out,” supposed that he spent that time going from garage to garage thereabout, inquiring for a yellow car. On the other hand, no garage man who had seen him ever came forward, and perhaps he had an easier, surer way of finding out what he wanted to know. By half-past two he was in West Egg, where he asked someone the way to Gatsby’s house. So by that time he knew Gatsby’s name.